A major obstacle to effective vaccination via antigenic plasmids is the need of the DNA vaccine to be delivered intracellularly. The delivery of naked DNA through a standard intramuscular injection is notoriously inefficient outside of rodent models. Historically, this has led to an inability to achieve robust immune responses in large mammals and humans. Several strategies have been developed to enhance the expression of DNA-based vaccines, such as codon-optimization, RNA optimization, leader sequence addition and the development of optimized consensus sequences. These optimization strategies can lead to improved, cross-reactive immune responses. The addition of co-delivered gene-based molecular adjuvants is another area where an augmentation of resulting immune responses frequently occurs. Despite the improvements in vector design and use of molecular adjuvants, there is still a clear need for an efficient method of administration of DNA vaccines that results in high level expression of the plasmid in the desired cell type of the desired tissue, most commonly, muscle, tumor or skin.
Drug delivery to dermal tissue (intradermal) is an attractive method in a clinical setting for a number of reasons. The skin is the largest organ of the human body, the most accessible, and easily monitored, as well as being highly immuno-competent. However, the impervious, barrier function of the skin has been a major obstacle to efficient trans-dermal drug delivery.
Human skin comprises approximately 2 m2 in area and is around 2.5 mm thick on average, making it the largest organ of the human body. Conventionally, the skin has two broad tissue types, the epidermis and the dermis. The epidermis is a continually keratinizing stratified epithelium. The outermost layer of skin is the stratum corneum (SC) and functions as the primary barrier. The SC is a 15-30 cell thick layer of non-viable but biochemically active corneocytes. The other three strata of the epidermis (S. granulosum, S. spinosum, S. basale) all contain ketatinocytes at different stages of differentiation as well as the immune Langerhans cells and dermal dendritic cells.
Both physical and chemical methods for trans-dermal drug delivery and gene delivery have been detailed by groups worldwide. Iontophoresis, lipid delivery and gene gun are such examples. A physical method to temporarily increase skin permeability is electroporation (“EP”). Electroporation involves the application of brief electrical pulses that result in the creation of aqueous pathways within the lipid bi-layer membranes of mammalian cells. This allows the passage of large molecules, including DNA, through the cell membrane which would otherwise be less permeable. As such, electroporation increases the uptake or the extent to which drugs and DNA are delivered to their target tissue.
Although the precise mechanism by which electroporation enables cell transformation has not been elucidated, a proposed theoretical model involves a poration event due to the destabilization of the membrane, followed by the electrophoretic movement of charged molecules into the cell. For electroporation to occur, the formation of pores requires that a threshold energy be achieved and the movement produced by the electrophoretic effect depends upon both the electric field and the pulse length.
In the case of DNA vaccines, electroporation has been shown to quantitatively enhance immune responses, increase the breadth of those immune responses as well as improve the efficiency of dose. More recently, the DNA-EP platform has been successfully translated into the human clinical setting and has demonstrated significantly improved immune responses in several vaccine studies. Therefore, there has developed a need for a dermal electroporation device that would be considered tolerable, user-friendly and easily amenable to mass production, while continuing to achieve high transfection rates resulting in robust immune responses.
Although a number of intramuscular devices have now successfully entered clinical trials, the procedure is generally considered invasive and painful. To be considered amenable to mass vaccination, especially in a pediatric setting, a solution for a more tolerable electroporation method is needed. Accordingly, an effective dermal electroporation device that is capable of delivering a multi-agent DNA vaccine in a tolerable manner is desirable.